In 2000, a small nonprofit in Chicago started testing a new approach to gang violence in the city's most dangerous neighborhood: They'd treat it like a contagious disease, using the same tools that public health specialists use to treat an outbreak of the flu or tuberculosis. A year later, shootings in the neighborhood had dropped 67%.

When researchers started to study the method—which quickly spread to other communities—they validated that the reduction in violence wasn't just a coincidence. In Baltimore, the same program was shown to reduce killings as much as 56%. In New York, a study found that the program reduced gun violence 20%.

The epidemiologist who started the original nonprofit (initially called CeaseFire, and now called Cure Violence), thinks that the same methods could be used to stem other types of violence as well—including mass shootings or violent extremism.

"The science behind this is that these are really different variations or different syndromes of the same problem," says Gary Slutkin, founder of Cure Violence and an epidemiologist at the University of Illinois at Chicago School of Public Health.

Slutkin spent a decade working in Africa, helping countries curb outbreaks of AIDS, tuberculosis, and cholera. When he returned to Chicago in 1995, friends started talking to him about the "epidemic" of violence in some of the city's neighborhoods. Slutkin looked at maps of the violence and noticed something: The patterns and clusters really did look like an outbreak of a contagious disease.

One study found "significant evidence" that both mass shootings and school shootings have a contagious effect.

Studies since then have pointed to the same idea for all types of violence. In 2015, one study found "significant evidence" that both mass shootings and school shootings have a contagious effect, making copycat events more likely for roughly two weeks. In Israel, another study found that students exposed to rocket attacks or terrorism were more likely than their peers to be violent themselves. Many more studies have similar findings.

Around the turn of the millennium, Slutkin began to wonder if public health techniques could be used to stop violence from spreading. Like an epidemic, the model starts by using trained workers to find people at the highest risk of "infection."

If the team learns that someone is likely to commit violence (if, for example, someone's family member has been shot and they're likely to retaliate), trained "violence interrupters" step in to try to talk them down. They also try to change community norms, just like public health workers in other contexts might convince people to wash their hands or use mosquito nets.

The methods worked, and like other public health interventions, a study showed they also spread beyond the people who were directly targeted. "Other people in that friend group of super high-risk guys changed their thinking about whether they would do violence under various circumstances," Slutkin says. "These were people who we hadn't been talking to. That's the same thing that happens with other behavior that we work on. Smoking behavior, we're talking to some people, but others start doing it, too."

Now, Slutkin's team has been presenting the methods to cities that hope to translate it to counter violent extremism. He thinks the motivations for violence are similar, whether someone is a gang member in Baltimore or an attacker in Nice.

"The problem is the contagious nature of violence and it being picked up by people who are susceptible, either by their proximity or having been traumatized a lot or socially isolated," he says. By committing the violent act, the person hopes to gain "some kind of credit, or belonging, or status."

"The problem is the contagious nature of violence and it being picked up by people who are susceptible."

Some responses, like hate speech against groups such as Muslims, obviously make things worse. "It does not cause people who are thinking in that direction to be less likely to do events," he says. "It causes them to be more likely to do events. They feel more justified, more isolated."

In Cure Violence's approach, workers try to intervene long before a deadly event occurs. It's a technique that could have been applied in other recent events. Omar Mateen, for example, who killed 49 people in Orlando, showed warning signs that could have triggered an intervention.

"He as a child was having a lot of difficulties," says Slutkin. "He was bullied, socially excluded, marginalized. He was involved with domestic violence. All of this stuff was not managed and treated, he became more and more marginalized, more and more susceptible. Then he comes to the attention of the FBI, and they're watching him. How long do you just watch somebody without somebody helping him? . . . Now that's not the job of the FBI. But if we were to be watching somebody, we help people."

As a doctor, Slutkin says, he takes a fundamentally different approach to violence. It's not about moralizing, but about saving lives. "It's a whole different lens than the current lens: 'bad people,' 'enemies,'" he says. "We don't have that in health. If you come to the hospital with chest pain, it's not that you're a bad person. It used to be different: We thought people with the plague or leprosy were bad people. People with seizures, it must be the devil."

It's not about moralizing, but about saving lives.

He thinks the lens needs to shift to giving people with problems the help that they need, ideally a long time before they even begin to contemplate picking up a weapon. It's something that may be less resource-intensive than some law enforcement. It can take as many as 12 agents to surveil a terrorism suspect around the clock.

"Therefore it really is impossible [to fully monitor all suspects]," he says. "Also no one's really being helped. It's just a matter of catching the person or not. Whereas with the health modality, one person can keep tabs on and be helpful to 15 or 20 people. If you have a group of 5 or 10 of these workers, you really can be interacting with a large part of a community."

Slutkin isn't arguing that law enforcement should have fewer resources. But cities need more balance from other groups that can try to help prevent crime from happening in the first place. "They're having to do too much," he says of law enforcement.

Some of the cities he's spoken with, in countries like Denmark or the U.K., are already trying to provide more services to help people at the greatest risk of violence. But, in general, cities everywhere aren't doing as much as they could. "We're behind," he says. "We're really behind."

Have something to say about this article? You can email us and let us know. If it's interesting and thoughtful, we may publish your response.

Never miss a story.

Sign up for the Co.Exist newsletter and get the best stories delivered to your inbox daily.